The present invention relates to a modulation technique which provides a high data rate signal, and an auxiliary data signal through a band limited channel.
It is always desirable to provide data at higher data rates through channels which have limited bandwidth. Many modulation techniques have been developed for increasing the data rate through a channel. For example, M-ary phase shift keyed (PSK) and Quadrature Amplitude Modulation (QAM) techniques permit compression by encoding a plurality of data bits in each transmitted symbol. Such systems have constraints associated with them. First, the hardware associated with such systems is expensive. This is because these techniques require a high level of channel linearity in order to operate properly. Consequently, extensive signal processing must be performed for carrier tracking, symbol recovery, interpolation and signal shaping. Second, such techniques are sensitive to multipath effects. These effects need to be compensated for in the receiver. Third, these systems often require bandwidths beyond those available in some applications (for example in-band on-channel broadcast FM subcarrier service) for the desired data rates.
It is also desirable to provide for several data signals through a channel. Some modulation techniques utilize the channel completely, while others leave some aspect of the channel unused. Frequency domain multiplexing and time domain multiplexing are two techniques for sharing a channel among a plurality of signals. By sharing the channel in this manner, the overall throughput through the channel is increased.
In accordance with principles of the present invention, a digital data modulator is coupled to a source of a digital data signal having a data bit period, and a source of an auxiliary data signal. An encoder encodes the digital data using a variable pulse width code having edges occurring in an interval within the data bit period. A pulse signal generator, generates respective pulses representing the edges of the encoded digital data signal. A carrier signal generator generates a carrier signal having carrier pulses corresponding to the respective pulses during the interval, and modulated by the auxiliary data signal otherwise.
In accordance with another aspect of the present invention, a digital data demodulator is coupled to a source of a modulated signal including successive bit periods, each having a first interval containing a carrier pulse, spaced relative to other carrier pulses, to represent a variable pulse width encoded digital data signal, and a second interval during which the carrier is modulated with an auxiliary data signal. A detector demodulates the modulated signal to generate a variable pulse width encoded signal in response to received carrier pulses during the first interval, and an auxiliary data signal during the second interval. A decoder decodes the variable pulse width encoded signal to generate the digital data signal.
The technique according to the principles of the present invention provides for a high data rate signal and an auxiliary signal to be simultaneously transmitted through a single channel. A system according to the present invention may be implemented using relatively inexpensive circuitry, is insensitive to multipath interference, and provides substantial bandwidth compression.